“They have clearly seen something moving. What it is, is not exactly clear.”” said Shep Doeleman, an astronomer at the Harvard-Smithsonian Center for Astrophysics and director of the Event Horizon Telescope that created the first-ever image of a monster black hole 55 million light-years from Earth in neighboring galaxy M87.

More than 50 years ago, scientists saw that there was something very bright at the center of our galaxy, says Paul McNamara, an astrophysicist at the European Space Agency. It has a gravitational pull strong enough to make stars orbit around it very quickly—as fast as 20 years, compared to our Solar System’s journey, which takes about 230 million years to circle the center of the Milky Way.

That “very bright something” was the Milky Way’s central supermassive black hole, Sagittarius A* (Sgr A*). Last October, 2018, before the release of the first image of the M87 black hole from the Event Horizon Telescope (EHT), astronomers announced that they found something orbiting the innermost possible orbit of the supermassive black hole, Sagittarius A* (Sgr A*), at the Milky Way’s center. Their measurements suggest that these “hotspots” — perhaps made of blobs of plasma — are spinning not far from the innermost orbit allowed by the laws of physics.

SGR A* has four million times the mass of our sun, which means that the black hole is about 44 million kilometers across. That may sound big, but for the telescope array on Earth some 26,000 light-years (or 245 trillion kilometers) away, it’s like trying to photograph a golf ball on the Moon. But, like a massive, dormant volcano, the Milky Way’s central black hole appears to be a sleeping monster.

Compared to some black holes, Sagittarius A* is much more anemic and fails to outshine a single bright star despite its comparatively enormous mass. But the data from the Event Horizon Telescope has opened a window on the inner workings of how material spirals towards black holes, finally disappearing across their event horizons, and growing into what the Perimeter Institute’s Avery Broderick calls “monsters lurking in the night.”

The newly detected hotspots, reports Joshua Sokol in Quanta, “afford astronomers their closest look yet at the funhouse-mirrored space-time that surrounds a black hole. And in time, additional observations will indicate whether those known laws of physics truly describe what’s going on at the edge of where space-time breaks down.”

“It’s mind-boggling to actually witness material orbiting a massive black hole at 30% of the speed of light,” marveled Oliver Pfuhl, a scientist at the Max Planck Institute for Extraterrestrial Physics.

ESO’s exquisitely sensitive GRAVITY instrument has added further evidence to the long-standing assumption that a supermassive black hole lurks in the center of the Milky Way. New observations show clumps of gas swirling around at about 30% of the speed of light on a circular orbit just outside its event horizon — the first time material has been observed orbiting close to the point of no return, and the most detailed observations yet of material orbiting this close to a black hole.

For astrophysicists, this glimpse at plasma is interesting in and of itself. “We have a totally new environment, which is totally unknown,” said Nico Hamaus, a cosmologist at Ludwig Maximilian University in Munich, who also developed the early hot spot theory.

ESO’s GRAVITY instrument on the Very Large Telescope (VLT) Interferometer has been used by scientists from a consortium of European institutions, including ESO, to observe flares of infrared radiation coming from the accretion disc around Sagittarius A*, the massive object at the heart of the Milky Way. Light from the four telescopes at the Very Large Telescope array in Cerro Paranal, Chile, can be combined to create, in effect, a single, enormous telescope.

The observed flares provide long-awaited confirmation that the object in the center of our galaxy is, as has long been assumed, a supermassive black hole. The flares originate from material orbiting very close to the black hole’s event horizon — making these the most detailed observations yet of material orbiting this close to a black hole.

While some matter in the accretion disc — the belt of gas orbiting Sagittarius A* at relativistic speeds — can orbit the black hole safely, anything that gets too close is doomed to be pulled beyond the event horizon. The closest point to a black hole that material can orbit without being irresistibly drawn inwards by the immense mass is known as the innermost stable orbit, and it is from here that the observed flares originate.

Relativistic speeds are those which are so great that the effects of Einstein’s Theory of Relativity become significant. In the case of the accretion disc around Sagittarius A*, the gas is moving at roughly 30% of the speed of light.

These measurements were only possible thanks to international collaboration and state-of-the-art instrumentation. The GRAVITY instrument which made this work possible combines the light from four telescopes of ESO’s VLT to create a virtual super-telescope 130 meters in diameter, and has already been used to probe the nature of Sagittarius A*.

Earlier this year, GRAVITY and SINFONI, another instrument on the VLT, allowed the same team to accurately measure the close fly-by of the star S2 as it passed through the extreme gravitational field near Sagittarius A*, and for the first time revealed the effects predicted by Einstein’s general relativity in such an extreme environment. During S2’s close fly-by, strong infrared emission was also observed.

“We were closely monitoring S2, and of course we always keep an eye on Sagittarius A*,” explained Pfuhl. “During our observations, we were lucky enough to notice three bright flares from around the black hole — it was a lucky coincidence!”

This emission, from highly energetic electrons very close to the black hole, was visible as three prominent bright flares, and exactly matches theoretical predictions for hot spots orbiting close to a black hole of four million solar masses. The flares are thought to originate from magnetic interactions in the very hot gas orbiting very close to Sagittarius A*.

The solar mass is a unit used in astronomy. It is equal to the mass of our closest star, the Sun, and has a value of 1.989 × 1030 kg. This means that Sgr A* has a mass 1.3 trillion times greater than the Earth.

Reinhard Genzel, of the Max Planck Institute for Extraterrestrial Physics (MPE), who led the study, explained: “This always was one of our dream projects but we did not dare to hope that it would become possible so soon.” Referring to the long-standing assumption that Sagittarius A* is a supermassive black hole, Genzel concluded that “the result is a resounding confirmation of the massive black hole paradigm.”

This research was undertaken by scientists from the Max Planck Institute for Extraterrestrial Physics (MPE), the Observatoire de Paris, the Université Grenoble Alpes, CNRS, the Max Planck Institute for Astronomy, the University of Cologne, the Portuguese CENTRA – Centro de Astrosica e Gravitação and ESO.